1. Field of the Invention
This invention relates to optical couplers, and, more particularly, to reducing the increase of numeric aperture (NA) in fiber optical couplers, including tapered-fiber-bundle (TFB) optical couplers.
2. Discussion of the Related Art
Applications that utilize high power light (optical) sources are diverse, ranging from machining and welding to pumping optical amplifiers and lasers. In these applications, a tapered fiber bundle (TFB) optical coupler is often used to couple multiple light inputs from multiple light sources into a single optical output port. [See, for example, DiGiovanni & Stentz, U.S. Pat. No. 5,864,644 (1999) and DiGiovanni & Tipton, U.S. Pat. No. 5,935,288 (1999), both of which are incorporated herein by reference.]
A schematic of a conventional TFB optical coupler 10 is shown in FIG. 1. A multiplicity of input fibers 11 (as shown in FIGS. 2A, 2B) couples input light Pin from a multiplicity of light sources 18 into TFB 10, which couples the combined light inputs to output port 12 and, optionally, to an output fiber 19 (FIG. 2D) spliced to output port 12. [For simplicity input light is shown entering only three input fibers 11 in FIG. 1, but, in practice, it is common to use more input in practice light typically is inputted to more fibers; e.g., seven (FIG. 2B) or nineteen (not shown). In addition, in typical prior art TFB configuration the fibers are bundled, with a central fiber being a single-mode signal fiber surrounding fibers being multimode pump fibers.]
As shown in FIG. 1, in a first section 15 the fibers 11 are bundled together and fused to one another, and light propagates separately in each fiber 11. In a second section 16, disposed in tandem with first section 15, the fibers 11 are not only bundled and fused (as in first section 15) but also tapered from a larger (maximum) equivalent diameter D17 (as shown in FIG. 2B) at interface 17 to a smaller (minimum) equivalent diameter D12 (as shown in FIG. 2C) at interface (output port) 12. The equivalent diameter is given by:Deq=Dfiber√n  (1)where n is the fiber of fibers in the bundle, and Dfiber is the diameter of an individual fiber. This formula assumes all of the fibers within the bundle have the same diameter. The ratio of the maximum equivalent diameter (D17) of the bundled, untapered fibers (FIG. 2B) to the minimum equivalent diameter (D12) of the bundled, tapered fibers (FIG. 2C) is known as the tapered-down-ratio (TDR), which is given by:D17/D12=TDR.  (2)Phrased another way, the TDR is simply related to the cross-section areas A17, A12 at interfaces 17 and 12, respectively, as follows:(A17/A12)0.5=TDR.  (3)The TDR typically has a value in the range of approximately 2 to 3. In second section 16, light is confined to a smaller cross-sectional area and will remain confined in the separate fibers only as long as the NA of the propagating light does not exceed that of the fiber waveguide. Thus, some portion of the light will spread among the fibers, at some point along the tapered second section 16. Therefore, the NA of light at the output port 12 is higher than at interface 17.
In some applications (e.g., optical amplifiers and lasers), one end of the output fiber 19 may, for example, be fusion spliced to second section 16 at interface 12, and the other end may be fusion spliced to a gain-producing fiber (GPF; not shown). The light output of the output fiber 19 is designated Pout. On the other hand, in other high power applications, such as cutting and welding, the high power light output of output fiber 19 may be applied directly to a work piece to be cut or welded.
One problem with conventional TFB couplers is that the tapering process increases the NA of the light at the output face 12 (as noted above), which, in turn, means that light rays emerging from output face 12 diverge more, which makes coupling into other devices (e.g., into output fiber 19) more difficult and less efficient. Although in an ideal TFB brightness would be conserved between the input and output, the numerical aperture of the light is still increased, which is undesirable for many applications.
Thus, there is a need in the art for a TFB optical coupler that minimizes the NA increase as light passes through it as compared to prior art TFB couplers of the type described with reference to FIGS. 1 and 2.